Conventionally, a swash plate type compressor is used in systems such as an air conditioning system of an automobile. According to a known swash plate type compressor, the transmission of motive power is carried out, as a swash plate and a piston reciprocate, thereby suctioning, compressing and discharging the gas. The swash plate is usually composed of aluminum or aluminum alloy and shoes, which make slideable contact with the swash plate when it rotates, are composed of iron or light weight ceramics such as alumina. The metal on metal contact at the shoe and swash plate interface requires special precautions to be taken in order to prevent undue wear and possible seizure of the shoe with the swash plate.
In a conventional swash plate compressor, the following problems are likely to occur. 1) The amount of oil contained in the refrigerant gas is decreased if the refrigerant leaks out of the swash plate type compressor. When the swash plate type compressor is operated under this state, lubrication at the sliding surface of the swash plate is decreased. In an extreme case, seizure of the shoe at the sliding surface of the swash plate occurs due to the generation of high temperature friction heat. 2) In the case where the compression of the liquid refrigerant takes place, the lubrication at the sliding surface of the swash plate is decreased. As a result, seizure of the shoe with the surface of the swash plate may occur.
Several methods have been developed to improve the lubrication at the shoe/swash plate interface and to lessen the wear of compressor swash plates. Conventional swash plates are treated with a tin coating to improve surface wear.
U.S. Pat. No. 5,655,432 treated the swash plate with a cross-linked polyfluoro elastomer bonded directly to the aluminum, a lubricious additive and a load bearing additive. The material is applied as a viscous fluid and is masked part in order to coat the component only at certain areas. The coating is also applied in a range of 13-50 microns and since the maximum allowed variation is only 10 microns the parts require machining after coating. The coating process itself adds to manufacturing complexity, and makes it more difficult to hold manufacturing tolerances than with a conventional tin conversion coating.
U.S. Pat. No. 5,056,417 treated the swash plate body with a surface coating layer made of tin and at least one metal selected from the group consisting of copper, nickel, zinc, lead and indium. While any of these five materials are alloyed with tin to improve its wear resistance, none of them are described as also acting to bind the coating to the swashplate.
The present invention discloses a tin/cobalt conversion coating with improved wear resistance and also excellent adhesion to aluminum or aluminum alloy surfaces which experience during use, sliding friction, e.g., surfaces of swashplates, pistons, etc. The coating retains the high-lubricity of tin on the aluminum swashplate. Thus, in the current invention, the added cobalt provides a tin/cobalt surface coating with improved adhesion over a conventional adherent coating tin conversion coating, which improves the wear resistance of the aluminum surface.